texts: Can expansion of space blueshift galaxies with respect to one another?

texts:
Suppose we have two galaxies in an otherwise devoid space, and suppose the galaxies are gravitating. Suddenly, dark energy kicks in, and accelerated expansion of space begins ...

(1D force diagram. Ruddy rectangles represent the two galaxies, violet ellipses represent dark energy.)

Because the galaxies are gravitating, dark energy between them expands space less, while "behind" the galaxies, where their mutual gravities don't overlap as much, dark energy exhibits a greater effect. The arrows above the galaxies compare force vectors. The black arrows are forces of gravitational attraction, the violet arrows are forces of expansion. Eyeballing the vectors' sum for each respective galaxy, it seems that the galaxies get an extra kick of acceleration from the regions of greater expansion behind them.

If we play the movie in our heads with these assumptions: the galaxies start off by accelerating toward one another under their own mutual gravitational attraction (each observes a blueshift), then dark energy kicks in and the galaxies accelerate with more energy than can be accounted for by acceleration due to their own gravities alone (blueshift observed much greater than it should be).

I have always read and heard of redshifted galaxies, not blueshifted ones (well, not so much, anyway), so something must be wrong here.

Question: Can the expansion of the universe drive two galaxies together like this? Does the net effect of expansion behind and between the galaxies cancel out, resulting in no greater acceleration? Have I got this all wrong?


texts: Why is there no color shift on the photo of the M87 black hole?

texts:
Last year, the first photo of a black hole in Messier 87 was published:

(Source: EHT)
It is quite obvious that about the lower half of the accretion disk is brighter.
This question (or rather, the asnwers) explain that it is caused by Doppler beaming. Since Doppler beaming partly constitutes of Doppler effect, I would expect to see a blueshift on the brighter areas and a redshift on the fainter areas; similarly to what the original rendering of the black hole from the movie "Interstellar" looked like:

(Source)
So why is this not the case for the photo of the M87 black hole?


texts: How could we detect planets orbiting black holes?

texts:
Detecting planets orbiting around stars can be made with several methods, transit photometry seems to be by far the most popular method nowadays.

I wonder how we could detect planets orbiting black holes. Which methods are we left with? Has it been done before?


texts: How to calculate longitude of Alcyone - η Tauri in the year 41050 BCE?

texts:
Using the following information from The Skylive, and simple arithmetic I tried to calculate longitude of Alcyone in the year 41050 BCE at a place: 29.9457 N , 78.1642 E.
Galactic Longitude 166.67°,
Galactic Latitude -23.45°
I could calculate it to be 330° against the expected value of 360°. I used J(2000) value of 25771 years for the precessional cycle of earth's axis.
Where did I miss?
Any help would be appreciated.


texts: Why look at an infrared telescopes's mirror with ultraviolet light? (Herschel Space Observatory)

texts:
While link-clicking for Where did Herschel Space Telescope go in 2013? I ran across the 2009 Time Magazine article Two Telescopes to Measure the Big Bang which shows the file photo below with the caption:

  Herschel's 3.5-m diameter mirror is checked with ultraviolet light at Europeís Spaceport in Kourou, French Guiana T VALLEE/AFP/Getty

According to Wikipedia's Herschel Space Observatory the telescope covers a wavelength range from far-infrared to sub millimeter waves; 55–672 µm.

Why then check the mirror's figue with a wavelength 200 times smaller in the ultraviolet? Even visible light would have been more convenient, and there should be several IR bands where coherent sources and imaging detectors work even in the humid French Guiana climate.


texts: Why is this shooting star wavy or squiggly toward the end of it's visible streak

texts: I was taking photos of some stars when I noticed this shooting star was wavy toward the end (start?) of it's trail. My cousin suggesting that it could be the CCD in my camera or as it spiralling as it enters the atmosphere. This is a heavily cropped image


texts:
Is there a site / service where I can get the data for the positions of satellites (I mean artificial satellite, ISS, etc.)?

I know that some sites provide a visualization of these positions, but I'm only interested in the data.

Thanks.

Edit, for clarifications: What I want is only the data. My final goal is to create my own site that displays the position of the satellite (I do that just for fun). That's why I just need the data, and not a service that displays the position on a map.


texts: Moving from computerised to normal amateur telescope?

texts: A few years ago I purchased a Celestron Nexstar 130 SLT computerised telescope thinking that it would be a perfect to get me into astronomy. The issue is the computer doesn't work at all and manually finding objects was a pain so I haven't used it in years. Now I want to get back into the hobby the 'proper' way by tracking objects with star maps and I want to get it right since photography and spectroscopy really excite me and that's what I want to end up doing. Despite all of this I can't seem to find a dovetail Equatorial mount that will fit my optical tube so now I feel I am stuck with a motorised mount that doesn't allow me to point my tube precisely and is really no fun. Is there any way for me to get into the hobby with my current optical tube using a different mount, use my computerised mount accurately enough for decent observation, or do I really have to buy a whole new optical tube just for the mount?


texts: Is it plausible that Mars could have been one of Jupiter's natural satellites at one time?

texts: Would more questions regarding the planet be answered if this was the case?


texts: What is the Maximum Speed that can be acheived Because of Acceleration Due to Gravity?

texts:
Planets and other bodies in the universe have different ranges of gravitational fields. What is the maximum speed that can be achieved because of acceleration due to gravitational field.

In other words which is the strongest and largest gravitational field discovered in the universe till date. What is the maximum speed An object accelerating in these fields can gain?


texts: Why are the magnetic poles of a pulsar so far off the rotational axis, yet stable?

texts:
My understanding is that a pulsar is a neutron star whose magnetic poles don't coincide with the rotational poles and whose magnetic poles sweep through Earth (allowing us to hear radio noise).

If we keep hearing pulsars, the polar offset must be very stable.

Why is such an alignment offset so stable?


texts: Why is frequency different for gravitational waves coming from SMBHs vs small compact binaries?

texts:
Why do gravitational waves emitted by supermassive black holes binaries have low-frequency range (~10$^{-9}$ - 10$^{-2}$ Hz) than those emitted by stellar mass compact binaries (~10$^{-4}$  - 10$^2$ Hz)?

Below, a NASA graphic showing the wavelength ranges of various phenomenon found here. Click for full size.


texts: Why is the solar wind most powerful near the heliosphere?

texts:
I've read this article, titled, "Voyager Mission Finds a New Type of Electron Burst at The Edge of Our Solar System". They state that,

Carried to the fringes of our Solar System by the shock waves of solar
eruptions known as coronal mass ejections, these energised particles
appear to accelerate even beyond the borders of our Sun's powerful
winds.
"The idea that shock waves accelerate particles is not new,"
notes astrophysicist Don Gurnett from the University of Iowa.
He says similar processes have been observed within the borders of our
Solar System where solar wind is most powerful.

I would think that everything slows down, maybe not so much in the vacuum of space, but why would these particles continue accelerating?


texts: I don't mean necessarily a granular chart of all of the different rings at different distances, but is it days? Weeks?


texts: How would water waves behave in partial gravity?

texts: Would an "indoor" pool of water on the Moon or on Mars behave differently than on Earth? Would the waves caused by a splash be higher or lower? Would they propagate faster or slower? Would boyance be different? And what would it be like to shower in slowly falling water?


texts: Satellite in orbit in front of and behind the Moon

texts:
Would it be possible to put a satellite into the same orbit as the Moon, but far enough ahead or behind the Moon to remain in place?

Has this ever been done in practice?


texts: Why is constant velocity drift not an accepted explanation of Hubble expansion?

texts:
Edward A Milne at the Oxford University proposed that what Hubble was
  observing was simply a natural sorting of random galactic motions. If
  a group of galaxies formed together moving at various speeds, it was
  natural that the fastest ones would now be the most distant, while the
  slow-moving ones would remain nearby.

Source: https://aeon.co/essays/how-they-pinned-a-single-momentous-number-on-the-universe

Why is this incorrect?

I understand that gravity wouldn't normally allow expansion at constant velocity, but we have already conjectured dark energy to account for that.


texts: How is spectroscopy used to deduce what an object is made of?

texts:
Spectroscopy is an analysis of light (or other EM wavelengths) that is often used by scientists to examine what an object is made from or contains.
Apologies if this is a stupid question, but this sounds effectively like saying “this thing is absorbing this wavelength or colour, therefore it is this substance”.
I’m confused as to how this works in practice. As a familliar example: light bouncing off leaves in summer can be a completely different colour to leaves in winter. Water when frozen looks to be a different colour to when it is liquid. Copper is shiny brownish unless it gets a bit rusty, then it’s bright green, etc etc.
Is it possible to explain how we can say with any amount of certainty that a given set of dips in EM wavelengths translates into identification of substances?


texts: How does V838 Monocerotis look today? Any subsequent light echo images, or is the party over?

texts:
This answer to the question V838 Monocerotis “light-echo” images morphed into nice video, but why so few original images? contains the following information below. There are other answers and more images there, so it's recommended to enjoy there as well.

Question: What does V838 Monocerotis look like today? Have there been subsequent light echo image sequences, or is the party over?

See also Why weren't the Hubble light-echo images of V838 Monocerotis supplemented by ground telescopes?

The V838 Monocerotis expansion (not a supernova) and the observation of the subsequent "spectacular" light echo was quite a notable event! From Nature 422, 405-408 (27 March 2003)

From Astronom. J. 135, 2, 2008 or ArXiv

  ".Galactic light echoes are extremely rare. The only other known example of extent similar to that of V838 Mon was the echo produced by Nova GK Persei 1901 (Kapteyn 1902; Perrine 1902; Ritchey 1902). Following early misunderstandings, light-echo geometry was properly described by Couderc (1939), and more recent discussions are given by many authors, including Chevalier (1986), Felten (1991), Sparks (1994), Sugerman (2003), and references therein".

It was the sole topic of an international conference photo from here:

Figure 2 of the Nature paper describes the preservation of the actual light curve (history) within the structure of the light-echo shell:

  "FIGURE 2. HST images of the light echoes
  The apparently superluminal expansion of the echoes as light from the outburst propagates outward into surrounding dust is shown dramatically. Images were taken in 2002 on 30 April (a), 20 May (b), 2 September (c) and 28 October (d). Each frame is 83" times 83"; north is up and east to the left. Imaging on 30 April was obtained only in the B filter, but B, V and I were used on the other three dates, allowing us to make full-colour renditions. The time evolution of the stellar outburst (Fig. 1) is reflected by structures visible in these colour images. In b, for example, note the series of rings and filamentary structures, especially in the upper right quadrant. Close examination shows that each set of rings has a sharp, blue outer edge, a dip in intensity nearer the star, and then a rebrightening to a redder plateau. Similar replicas of the outburst light curve are seen propagating outwards throughout all of the colour images."

also see: This is an artificially constructed animation, morphing eight existing images to suggest what a higher cadence imaging campaign might have seen: https://www.spacetelescope.org/videos/heic0617a/


texts:
Using the NED, you can search via frequency range corresponding to the selected pass band filter. I'm a beginner here and don't really understand. Here's the quote:

NOTES: Objects with observations in the frequency range corresponding to the selected passband filter will be returned. For
example, selection of Spitzer MIPS 24 µm will also return objects with
photometry from IRAS 25 µm because these filters overlap. This feature
permits the location of objects with photometry in the selected region
of the spectrum, while cutting across multiple surveys, detectors and
filters that have been cross-correlated by NED. A future upgrade will
include an option to restrict the search to only the selected passband
filter.

There are then a list of 'passbands', all the way up to gamma. What is this actually mean? The amount of radiation given off by a celestial body?
I'm looking at the categories I'm more familiar with:
Helio. Radial Velocity
Redshift
Major Diameter (arcmin)
Minor Diameter (arcmin)
Magnitude and Filter
Classifications
And I'm not sure where this radiation fits in. Is it
Spectral Energy Distribution (SED)

? Could someone explain the following terms to me if this is the case:

Observed Passband
Uncertainty Units
Freq (Hz)
Mode Aperture/Qualifiers
Reference Code   No.

Wnad what about published and homogenised variables? Are homogenised variables the avarage of all observations?


texts: What were they expecting to see when Halley's comet appeared in 1910?

texts:
This answer to Any record of the Earth passing through the tail (not trail) of a comet? mentions

Earth passed through the tail of Halley's Comet in 1910. It caused a bit of a panic due to claims that cyanogen detected in the tail would cause harm: https://en.wikipedia.org/wiki/Halley%27s_Comet#1910

and links to the February 8, 1910 New York Times item Comet's poisonous tail; Yerkes Observatory Finds Cyanogen in Spectrum of Halley's Comet
This comment reminds us that its appearance was not unanticipated:

Astronomers of that time certainly knew when to expect Halley's Comet. After all, this was its third appearance after Edmond Halley's famous prediction. It seems reasonable to think that Wells & much of the general public would have also known it was coming.

And this comment reminds us:

H G Wells wrote "In the Days of the Comet" in 1906, in which the earth passes through the tail of a comet. I don't know if astronomers of the time had predicted the Halley event 4 years early: In the Days of the Comet

Question: What exactly were astronomers expecting prior to the comet's arrival? Did they have a predicted orbit, time of appearance and closest distance to Earth and estimated brightness prepared? If so, how close did the predicted behavior match the actual?
Was there any prediction or concern about the Earth passing through the comet's tail?

 (click for larger)


texts: Size and Mass of a typical "small" asteroid that impacts the Earth?

texts:
I'm looking for some realistic data about the size (in meters) and mass (in kg) of a typical small asteroids that impact the Earth, without disintegrating in the atmosphere.

Any suggestion ?


texts: How to remove radial velocity effects of an exoplanet from the host star spectra?

texts: I have a spectra of an M dwarf and I would like to remove the Doppler shift effects due to a known exoplanet, with known planetary parameters, from the whole spectra of the star. I have shifted/corrected the spectra for the radial velocity of the star itself along the line-of-sight but I'm not sure how to shift/correct it for the wobble of the star around a common centre of mass due to a planet orbiting it. Can someone please help?


texts: If two pieces of the same type of metal touch in outer space, they will bond together permanently?

texts:
Is this true? And if so, why?
http://www.snapple.com/real-facts/cap-view/1003


texts:
I do know nothing can escape a black hole, and I understand popsci explanation of geodesics of space-time around and inside the event horizon.

I however have some trouble with following thought experiment, given there are many references that crossing event horizon of SMBH should be totally unnoticeable for the subject falling through:

I am at gym - a regular one, on Earth. Escape velocity of Earth is around 11.2 km/s. Yet I can lift weights up say one meter even at slower speed, say one meter per second.

Next, consider me hovering one meter above event horizon. As far as I know, for non-rotating BH this is totally possible. Similar to earth, I could lift weight up one meter at speed less than escape velocity (which at this point is speed of light minus x).

Now, I’m hovering one meter below the event horizon. Would I not be able to lift the weight up one meter at speed less than escape velocity (which at this point is speed of light plus y; not sure if x and y are equal), or would I not be able to hover at this position?

Assuming answer is no to either or both, how this would appear to me as the subject?


texts: How I can bin a magnitude phase-folded lightcurve? I take mean values magnitudes and it's uncertainties but this is wrong.


texts: Calculate mass of exoplanet from transit method

texts:
Is there any way to calculate (or at least estimate) mass of an exoplanet from transit method? I know that mass can be calculated with radial velocity method, however I would like to create program for processing Kepler light curves (without radial velocity data).
The data I have are:

radius, mass, surface temperature and luminosity of star,
orbital period, semi-major axis and radius of exoplanet,
duration and depth of transit.


texts: How is the average distance between 2 objects orbiting around a third object calculated?

texts:
I want to know the average distance between Io and Europa knowing that Io's semi-major axis (around Jupiter of course) is 421,800 km and Europa's semi-major axis is 671,100 km.
At first I thought it's the average of the closest approach (671,100 - 421,800 = 249,300 km) and the farthest approach (671,100 + 421,800 = 1,092,000 km), but the average of those is exactly 671,100 km, which is Europa's semi-major axis, and I find that hard to believe.
Assuming the orbits are co-planar and perfectly circular, which they more or less are, surely there must be a simple formula to calculate the average distance?
I asked this on another site and the answer was "run a simulation!", which is a very good answer, and I would've already done that if I knew how lol.


texts: Coordinates of known black holes in our Galaxy (Milky Way)

texts:
There are some articles in wikipedia with the list of known (or candidates) black holes here and list of nearest black holes. Also there is a map of Milky Way.

My question is where can I get approximate position of the known black holes in our Galaxy (Milky Way) as Galactic coordinate system or maybe some other galaxy coordinates?


texts: What is the most extreme weather found on another planet?

texts: On Venus, there is really inhospitable weather, as well as within the gas giants in our solar system. Are there examples of even more extreme weather on planets found in other solar systems than ours?


texts: Moonrise after sunrise but moonset before sunset?

texts:
Today (1/24/2020) for Boston MA timeanddate.com lists the following four times:

Sunrise  7:05 AM
Moonrise 7:06 AM
Moonset  4:33 PM
Sunset   4:47 PM

Now, if moonrise is after sunrise, I'd have thought the "new moon" conjunction already happened. But since moonset is before sunset, I'd think it hasn't happened yet.

What gives? Is this a rounding error? Usually the moon "moves" across the sky "slower" than the sun does (as in a solar eclipse where the moon moves from west to east back across the sun).


texts: Visible satellites real time position API

texts:
I'm trying to obtain the azimuth and elevation of all (or at least the majority) visible artificial satellites from a given latitude and longitude. Is there a quite stable API that does that for free? I found some APIs that track particular satellite event, ISS passes and so on, but nothing about what I'm looking for.

This should run on an ESP8266 board, so I can't afford to store positions database or a large ephemeris and do to calculations on it.


texts: How to calculate the maximum and minimum solar azimuth at a given location?

texts:
Every Azimuth equation I have been able to locate so far depends on inputting a time, and I do not want to have to brute force the solution by simply trying all of the times. (See below for standard equations.)

My use case doesn't require great precision, within a few degrees is fine. So, ideally I'm looking for equation(s) that take only latitude (and possibly longitude?) as input and output max and/or min azimuth angles for that location.  Also, I'm only going to care about years within a century of now so I don't believe that the year to year variation will matter given my precision requirements, but if I'm wrong I'm happy to provide the current year as well.

Standard Solar Equations Based on NOAA's Solar Calculator

Note: The sunrise and sunset results are theoretically accurate to within a minute
 for locations between +/- 72° latitude, and within 10 minutes outside of those latitudes.
 However, due to variations in atmospheric composition, temperature, pressure and conditions,
 observed values may vary from calculations.
...
Please note that calculations in the spreadsheets are only valid for dates between 1901 and 2099,
 due to an approximation used in the Julian Day calculation."

$\Xi = $Latitude (+ to N)

$\Phi = $Longitude (+ to E)

$\omega = $Time Zone (+ to E)

$d = $Date

$\tau = $Time (hrs past local midnight)

$\upsilon = $Julian Day = $d + 2415018.5 + \tau - \frac{\omega}{24}$

$\sigma = $Julian Century = $\frac{\upsilon - 2451545}{36525}$

$\rho = $Geom Mean Long Sun (deg) = $\left(280.46646 + \sigma
\left(36000.76983 + 0.0003032\sigma\right)\right) \bmod 360$

$\xi = $Geom Mean Anom Sun (deg) = $357.52911 + \sigma\left(35999.05029 - 0.0001537\sigma\right)$

$\mu = $Eccent Earth Orbit = $0.016708634 - \sigma\left(0.000042037 + 0.0000001267\sigma\right)$

$\lambda = $Sun Eq of Ctr = $\sin\left(\xi^{rad}\right)\left(1.914602 - \sigma\left(0.004817+0.000014\sigma\right)\right) + \sin\left(2\xi^{rad}\right)\left(0.019993 - 0.000101\sigma\right) + 0.000289\sin\left(3\xi^{rad}\right)$

$\kappa = $Sun True Long (deg) = $\rho + \lambda$

$\iota = $Sun True Anom (deg) = $\xi + \lambda$

$\theta = $Sun Rad Vector (AUs) = $\frac{1.000001018\left(1 - \mu^2\right)}{1 + \mu\cos\left(\iota^{rad}\right)}$

$\eta = $Sun App Long (deg) = $\kappa - 0.00569 - 0.00478\sin\left(\left(125.04-1934.136\sigma\right)^{rad}\right)$

$\zeta = $Mean Obliq Ecliptic (deg) = $23 + \frac{26 + \frac{21.448 - \sigma\left(46.815+\sigma\left(0.00059 - \sigma0.001813\right)\right)}{60}}{60}$

$\epsilon = $Obliq Corr (deg) = $\zeta + 0.00256\cos\left(\left(125.04-1934.136\sigma\right)^{rad}\right)$

$\delta = $Sun Declin (deg) = $\left(\arcsin\left(\sin\left(\epsilon^{rad}\right)\sin\left(\eta^{rad}\right)\right)\right)^o$

$y = $var y = $\tan\left(\left(\frac{\epsilon}{2}\right)^{rad}\right)^2$

$\Gamma = $Eq of Time (minutes) = $4\left(y\sin\left(2\rho^{rad}\right)-2\mu\sin\left(\xi^{rad}\right)+4\mu y\sin\left(\xi^{rad}\right)\cos\left(2\rho^{rad}\right)-0.5y^2\sin\left(4\rho^{rad}\right)-1.25\mu^2\sin\left(2\xi^{rad}\right)\right)^o$

$\gamma = $True Solar Time (min) = $\left(1440\tau + \Gamma + 4\Phi - 60\omega\right) \bmod 1440$

$\beta = $Hour Angle (deg) = $if\left(\frac{\gamma}{4}<0\right) \{\\
\frac{\gamma}{4}+180\\
\} else \{\\
\frac{\gamma}{4}-180\\
\}$

$\Omega = $Solar Zenith Angle (deg) = $\left(\arccos\left(\sin\left(\Xi^{rad}\right)\sin\left(\delta^{rad}\right)+\cos\left(\Xi^{rad}\right)\cos\left(\delta^{rad}\right)\cos\left(\beta^{rad}\right)\right)\right)^o$

$\alpha = $Solar Azimuth Angle (deg cw from N) =
$if\left(\beta>0\right) \{\\
\left(\arccos\left(\frac{\sin\left(\Xi^{rad}\right)\cos\left(\Omega^{rad}\right)-\sin\left(\delta^{rad}\right)}{\cos\left(\Xi^{rad}\right)\sin\left(\Omega^{rad}\right)}\right)^o+180\right) \bmod 360\\
\} else \{\\
\left(540-\arccos\left(\frac{\sin\left(\Xi^{rad}\right)\cos\left(\Omega^{rad}\right)-\sin\left(\delta^{rad}\right)}{\cos\left(\Xi^{rad}\right)\sin\left(\Omega^{rad}\right)}\right)^o\right) \bmod 360\\
\}$


texts: Can my Sky Watcher Infinity 76P "make the sky bright" & do I want it to?

texts:
I've seen quite a few time-laps films where the Milky Way is seen as bright and filling the sky, as some fantastic glowing cloud full of glittering stars.

I wonder if it is possible to see this (or something a little similar) through a telescope using my eyes (not a camera)?

It seems to me that you would want as little magnification as possible to see the sky like this, and it would be all about gathering as much light as possible? This answer describes this scenario as "Low Brightness" (and low resolution) objects.

I have a Sky Watcher Infinity 76P telescope. At the moment, it has a 30x erecting lens that tends to wobble in its helical mount, and doesn't give a good image. I've read claims that with a non erecting lens and a bit of care, this telescope can give good images.

Are non magnifying lenses available for the Sky Watcher? Or would I be better off taking a different approach completely, such as buying binoculars?
I'd like to be able to share this with my children, and they find the existing Sky Watcher lens hard to use – as well as its wobble, I think this is because it has a narrow "exit pupil" (?), so that you need to get your eye in just the right place. Can I get a low magnification lens that is more forgiving for the user?
Can I expect to be able to see something from my backyard in a small UK town? Some of the shots in that film I link to seem to show clear bright sky pictures when there are other sources of light on the ground, or even the setting sun.

Many thanks.


texts: Conversion from Equatorial Coordinate to Horizon Coordinates

texts:
I'm developing my own Planetarium software using C++ following the book Practical Astronomy with your Calculator or Spreadsheet 4th Edition.

I don't know if I my formulas are correct or not, but when I check my results (Azimuth and Altitude) with Stellarium, they are different.

I have also checked my values with this page, but they are different. By the way, the azimuth and altitude given in that page are also different from the values given in Stellarium.

Do you know where can I find the formulas to covert celestial coordinate to horizontal coordinates?


texts: Latest sunrise/earliest sunset out of phase with solstice?

texts:
Everybody knows that the shortest and longest days of the year occur on the solstices.  However, examination of sunrise/sunset tables shows that the maxima and minima of sunrise/sunset times are offset from the solstices by about 10 days.

According to timeanddate.com for my location (Portland, OR), for the winter solstice, the earliest sunset is about December 10 (16:27), and the latest sunrise is around January 1 (07:51).

This is counterintuitive... what's the explanation?


texts: How can I estimate the M/L ratio from a fitted SED on a galaxy?

texts:
This answer (here in the forum) provides background to my question: https://astronomy.stackexchange.com/a/5953

I am trying to understand the process in which a stellar population synthesis (SPS) is created, in order to calculate the stellar mass of a galaxy. (This is my main goal: understand how the stellar mass is calculated via SPS and broadband photometry)(Section 2.5:  https://ned.ipac.caltech.edu/level5/Sept14/Conroy/Conroy2.html)

Ok.
I have already understood the process to obtain a stellar population library (SPL). Such SPL will be fitted to observational data in order to create a SED (Spectral Energy Distribution)

Ok. Now I have the fitted SED on a galaxy, here are my questions:

How can I estimate the M/L ratio of that galaxy?
How can I estimate the L?

I feel it is an easy and direct answer, but I don't get it.


texts: Is the Moon not in hydrostatic equilibrium?

texts:
Given the recently-announced observations from VLT/SPHERE that 10 Hygiea may be sufficiently round to qualify as the second main-belt dwarf planet, I found myself perusing Wikipedia's  Hydrostatic Equilibrium article, and came across the following passage, which currently has no linked references:

  The smallest body confirmed to be in hydrostatic equilibrium is the dwarf planet Ceres, which is icy, at 945 km, whereas the largest body known to not be in hydrostatic equilibrium is the Moon, which is rocky, at 3,474 km.

I'd long assumed that the Moon was in hydrostatic equilibrium, given its spherical-to-the eye shape and ranking among the 20 largest solar system objects. If the quoted statement is true, why isn't it in hydrostatic equilibrium?


texts:
For a body to qualify as a planet according to the IAU definition it must have "cleared its neighborhood". What evidence is there Mercury indeed cleared its neighborhood? Perhaps it migrated there afterwards, when the neighborhood had already been cleared. Does the Grand Tack hypothesis impact our definition of the inner planets as planets?
https://en.m.wikipedia.org/wiki/IAU_definition_of_planet
See Grand Tack Hypothesis.
The present definition of a planet is vulnerable as it seems connected to a model of formation of the solar system. The answer below states that in practice an operational definition used that I believe is adequate.


texts: Assuming a sufficient amount of mass above the density threshold, does the actual concentration of the mass matter in creating a black hole?

texts:
I've heard that the actual density of a black hole (within the volume of the event horizon) is pretty low - does that mean that any sufficiently large volume of mass over that density is also a black hole? Or does the actual concentration of mass within the event horizon matter?

Forked from "Is it possible that a ultra-large portion of the space we live in is already inside a black hole? How could we refute this?"


texts: How many earth-like planets do we know at this moment?

texts:
Well, the title says everything...
How many earth-like planets have we discovered out there?
Discard too big (made of gas), too cold, too hot, etc.
Consider earth-like sun distance, size, temperature, the rocky ones...


texts:
Press-Schechter formalism allows one to universally quantify the number of massive dark matter halos in a given volume -- The Halo Mass Function. The density contrast is given by the rms mass variance in Fourier space, which is the dependent on the linear power spectrum $P_{\rm lin}(k,z)$ in a smooth window $\widetilde{W}(R;k)$
\begin{align}
\sigma^{2}(R,z)
= \frac{1}{2\pi^{2}}
\int_{0}^{\infty} dk\ k^{2} P_{\rm lin}(k,z) \left| \widetilde{W}(k;R)\right|^{2}
\end{align}

The halo count can be scaled via observations in a defined window region of $R = 8\ h^{-1}\rm Mpc$. We can solve for this analytical/numerically and measure it from CMB fluctuations, given by the parameter $\sigma_{8}$. The power spectrum, as well as the halo mass function, is then scaled through the normalization of the analytical variance and the observed variance.

My question is then, why do we choose a smoothing radius of $R = 8\ h^{-1}\rm Mpc$? Is it arbitrarily chosen to compute the number variance in a cluster, or is their more too that? What prevents me from defining the window radius of $R = 6\ h^{-1}\rm Mpc$ (just ignore the fact that the  WMAP and PLANCK results give $\sigma_{8}$ in the radius of $8$)?


texts: How does the earth's plane of orbit (ecliptic) vary over time?

texts: Does the earth's orbital plane change with time relative to the distant stars? Does the Sun follow the same path through the zodiac (ignoring proper motions of the zodiacal stars)?


texts: How is interstellar gas density mapped from GAIA data?

texts:
I found the image below in Space.com's article This 3D Color Map of 1.7 Billion Stars in the Milky Way Is the Best Ever Made.

The caption for this image reads:

  The Gaia spacecraft gathered observations for this all-sky view of the Milky Way and neighboring galaxies between July 2014 and May 2016, releasing the data on April 25, 2018. This image shows all the stars' colors and brightness (top), the total density of stars (middle) and the distribution of interstellar gas and dust across the galaxy (bottom). Credit: ESA/Gaia/DPAC

The first two maps reflect data obtained from analysis of GAIA's measurement of starlight as explained in this answer, but the third map is described as "the distribution of interstellar gas and dust across the galaxy."

As far as I understand, GAIA does not transmit all complete image data to Earth, but instead does a significant amount of image processing, object detection, classification and analysis, and other data reduction in order to extract positions and radial velocity measurements for point-like objects such as stars, though of course small solar system bodies such as comets and asteroids end up in the raw data as well.

Question: How is interstellar gas density obtained from GAIA data?


texts:
From the time that the sun appears on the horizon, or meets it on its setting, to the time that it is fully visible, or no longer visible on its setting, how much time passes?
Secondly, is there a place in the world where a sunrise/sunset occurs over a period of a few days? Meaning, that from the time it begins to appear over the horizon until it is fully visible, a period of a few days pass without night intervening (and the same for the opposite with sunset)?


texts: Are there pictures from a solar eclipse of the earth taken from a moon landing unit or orbiter?

texts: Did any moon landing vehicle or orbiter took pictures of a solar eclipse happening on the earth, i.e. the moon's shadow on the earth? (Alternatively from other spacecraft showing the whole globe). A link to those pictures would be nice.


texts: Was Newton the first to mention the orbital barycenter?

texts:
A barycenter is the common center of mass of an orbiting system. Here is a illustrative gif from wikipedia:

The first mention of something like a barycenter that I could find is in a translation of Newton’s Principia, published in 1687, proposition 12, theorem 12:
https://en.wikisource.org/wiki/The_Mathematical_Principles_of_Natural_Philosophy_(1846)/BookIII-Prop2.
This includes the quote that: “the common centre of gravity of Jupiter and the sun will fall upon a point a little without the surface of the sun” which to me demonstrates a clear understanding and articulation of the idea of a barycenter. Does anyone know of an earlier clear reference to a barycenter?
Related: What are Kepler's laws (as he wrote them)?


texts:
Rogue planets can be designated as rogue planets due to one of two possible scenarios: 1) the "rogue planet" formed as a sub-brown dwarf, or 2) the planet was ejected from its home star system. I want to make it clear that here, I'm interested in objects of the second category - objects that are similar to the planets we find in our solar system.

Could a rogue planet, having been ejected from its home star system, be gravitationally re-captured by another star system? If so, would it be likely to disrupt other bodies in the stellar system (I know this depends on size; let's say that the rogue planet is a "Hot Jupiter")?

I realize that we don't know a whole lot about these objects (let's face it, we only know of a handful), so I'm not looking for examples, but any logical answers would be pretty helpful.


texts: Are the stars in all galaxies orbitting around a middle?

texts: In our Milkyway all the stars are orbitting around the black hole in the center. But what is the cause it this rotation and are there galaxies where the stars aren't orbitting a center?


texts: How does one compute the right ascension and declination of the Moon at a given time (or for that matter, the altitude and azimuth)?


texts: Why isn't most hydrogen in the universe molecular (diatomic), instead of atomic (monoatomic)?

texts:
Similar questions have been asked before; but, why?
Is the monoatomic hydrogen left over from the Big Bang?  And hasn't had the opportunity to collide with other hydrogen atoms yet?
Or are hydrogen molecules that do form split up after absorbing ultraviolet radiation or X-rays?  Which occurs more frequently than molecule-forming random collisions?
What is the 'activation energy' needed to form diatomic hydrogen from atomic?  Is it too much for the randomly-colliding hydrogen atoms to form molecules in the depths of space?
I mean, if the loose protons and electrons shot out by stars can (usually) find each other to form atoms, which is why neutral atomic hydrogen is more common between the stars and planets than ionized hydrogen, why can't the lonely atoms find each other to form ${\rm H_2}$?


texts: For example: Are they soft landed meteorites? Or do they form from melted material during a violent impact? Or are they the result of some kind of erosion process?


texts: How to write a normalized, dimensionless form of Binet's relativistic equation?

texts:
$$\frac{\delta^2 u}{\delta \theta^2} + u = \frac{\mu}{h^2} + 3 \mu u^2$$

This is Binet's relativistic equation. Here $u = 1/r$ where $r$ is the radial distance, and $\mu$ is the reduced mass of the Sun and Mercury.

I am trying to calculate the perihelion precession path of Mercury numerically in Python and for doing so I would like to use a dimensionless form of this expression.


texts: What makes the Earth infinitely rotating around its axis

texts:
I was reading two sites before I posted the question here. One site says something like that the collisions in Universe eventually set the planets rotating, another site says that our planet is formed from some collapsing Nebula or whatsoever and Nebula was rotating when it was about to die and that set the Earth rotating at first place.

The question should be probably better specified. Is Earth rotating because of some continuous force, or because at first place there was some one-time force which set it rotating and the effect lasts until now?

As for the first case, which force is it? As for the second case, that should break our physical laws, because there would be one force, applied a very long time ago which is still in effect and not about to slow down and with a very very long lasting effect. Because thousands or millions of years for rotation is a very long time, isn't it. Or is it slowing down?

BTW: the infinite movement around orbit is bit mysterious too, because you can say that's because of gravity, but gravity itself attracts the things only to one direction, but orbit movement is like parallel with the gravity emitter, and again it is perpetual.


texts: Can equatorial mounts be used at lower latitudes?

texts:
I wanted to order a Celestron 127EQ and I was going through the manual online and just noticed that the manual says that latitude is adjustable in about a 20-60° range.

The PowerSeeker equatorial mount can be adjusted from about 20 to 60 degrees (see figure 4-3)

I'm in Costa Rica which means that it's less than 10° latitude. Is the 20-60 limit a hard limit or it can be lower?
By looking at various telescopes equatorial mounts pictures in internet, I think all will have the problem that the counterweight might hit the legs of the tripod if lowering the angle a lot, so it's not like a problem unique to 127EQ.
I have seen that a cellphone can be used to measure this angle so that would not be a problem as long as the mount can physically rotate down to about 10° but I wonder if this setup would not trip the telescope because the center of mass would be too far to the front..... at least while setting up its axis.


texts: How do we get the angular size of a crater without lens?

texts: Suppose I want to calculate the diameter of the crater. I apply the small angle formula $$diameter=\frac{\theta_{diameter}}{206265''}\times D$$ where $\theta_{diameter}$ is the angular size of the crater, in arcseconds, and $D$ is the average surface-to-surface distance from the Earth to the Moon, which is around $375,900$ km. We know that the moon is about $\frac12$ degree, which is $1860$ arcseconds. But how do we estimate the angular size of the crater without using lens, with only  eyes? I was thinking using ruler and applying scale but that way, I think, is inaccurate because the Moon has a round surface.


texts: How can a Grade-11 Student avail TESS data and learn to process?

texts: As mentioned, I am a Grade-11 student. I have always been interested in Data Processing activities. This sums up my experience of working on Asteroid hunt Campaign under IASC (International Asteroid Search Campaign). I yielded more than 22 Preliminary discoveries at once. Now I wish, to hunt for KBO (Kuiper belt objects) and Exoplanets. Definitely, hunting exoplanets is not a kids' task, but I possess the ability to learn and process and render astronomical data under guidance! I am ready to do everything. I am more inclined towards searching Outer-Worlds than the inner objects! Mentor-ship in this regard will be gratefully appreciated. I have searched quite a few things but none helped. I'm a Zooniverse user too. And over there, project slots were filled by other Citizen-Scientists!


texts: Delete key does not always work in IRAF's xgterm on Mac

texts:
Sometimes it does not let me delete text in an xgterm terminal and outputs ^H instead.

Example:

Run IRAF:

xgterm -e cl

It opens the xgterm, there I type:

splot black_hole.fits[0]
Image line/aperture to plot (0:) (12): 192

Then I realise I made a mistake and press the Delete key on Mac, and it writes the ^H text instead:

Image line/aperture to plot (0:) (12): 192^H^H

This is very annoying. Is there a way to fix it?

Note that delete key works in xgterm initially, but breaks and writes ^H in the case above.

I've installed IRAF 2.16 through astroconda iraf-all package on Mac.

Update

The only solution I found was to use PyRAF instead. PyRAF uses the normal Mac's terminal, which does not have those Delete key issues. I can still run the same IRAF commands in PyRAF terminal, so it works fine for me.


texts: I know this question had often been asked on Physics.SE as well as this site but I don't get them. What do mean when we say that space is expanding? I mean that space is no physical entity that can expand. Space is just (apparently) "nothingness". Please solve my confusion.


texts: Relationship between photometric colour and redshift, part two

texts:
In the graphs in my previous question here, there are negative-sloped regions as the colour-z relationship oscillates. So if I look at how the I-W2 and W3-U colours change with redshift, as I've done here:

with I-W2 and W3-U both getting lower (bluer, is that right?) does that mean that my graphs indicate one of the upward-sloping regions?

What else does the redshift evolution of I-W2 against W3-U indicate?


texts: Difference between 3D real space and 3D redshift space?

texts: With regards to cosmological simulations (e.g., N-body dark matter only simulations), I've seen people talk about the positions of objects (halos, galaxies) in the simulation in terms of 3D real space and 3D redshift space. The idea is that the positions of objects in 3D real space does not add the contribution of redshift space distortions, whereas 3D redshift space does. But since redshift space distortions are a real physical effect (i.e., the positions/velocities of objects will be affected not only by cosmological expansion but also gravitational interactions), why is 3D real space ever even considered? Is it some kind of highly idealistic cosmology-dependent only scenario, ignoring peculiar velocities?


texts: A few questions regarding the transit of planets

texts:
Here we will be concerning ourselves with the transit of a planet with it's parent sun.

I have a few questions that I am not sure about:

1) Do all planets transit their parent sun? It would seem to me that if all planets "cross" their parent star in some form, then depending on our point of view, we should be able to see the transits. For example, we never see a Mars transit, but if we alter our point of view to be farther then mars, then we would.

2) If all the planets are in the same orbital plane, would the transit of one, imply the transit of all the others? For example, our solar system is relevatily in the same plane, and it seems like Mercury/Venus transit, would also imply earth and beyond transits. We just have to be in right perspective.

3) If a planet transits its star, does the probability of a transit increase/decrease/same if planet semi major axis changes? I am not sure about this one. I still think that it would be 100% the fact that a transit will occur, but its more unlikely that we would ever see it, because we need our angle to be just right.

4) If a planetary system has multiple planets, would we detect transits from several planets if they are all in the same orbital plane, or if they have different orbital planes? This kind of builds off of #2, and I think the answer is the same orbital plane, because the "different" orbital planes could be such that are 90 degrees from our point of view, meaning we would never see the transit.

I want to know if my understanding of transits is correct by these questions. Thanks!


texts: Can a tidally-locked planet maintain an obliquity

texts:
If a planet is tidally-locked, can it have an obliquity?

e.g. the Earth's rotation is being slowed by tidal forces.  A long way in the future it will be tidally locked.  As it slows, will the obliquity also be eroded or will other forces (such as the orbit of the moon) allow it to maintain an obliquity?


texts: Why does a particle feel no force at radii greater than itself?

texts:
I am working through Liddle's "An Introduction to Modern Cosmology", and in a newtonian derivation of the Friedman equation he states:

  in a spherically symmetric distribution of matter, a particle feels no force at all from the material at greater radii, and the material at smaller radii gives exactly the force which one would get if all the material was concentrated at the central point.

He attributes this result to Newton, Can anyone give a mathematical or physical explanation as to why this is true?

Thanks


texts: Why the Sun-Jupiter barycentre is outside Sun?

texts: The Earth-Moon barycentre is said to be inside Earth. But Moon-Earth mass ratio (1.2%) is larger than Jupiter-Sun mass ratio (0.09%). Then how come the Jupiter-Sun barycentre is outside the surface of Sun?


texts: What Crayford focuser to buy for a B&L Criterion 8000?

texts:
I'd like to get a dual-speed Crayford focuser for my newly acquired a B&L criterion 8000 (8") as the mirror shift is pretty bad.
I'm really not sure which one to buy because I'm quite confused about the dimensions and potential adapters I need, and don't know how standard this config is.
Let's call [A] the 4 screws-piece at the output of the optical tube, and [B] the threaded adapter that I'm holding in my hand in the 3rd picture. The ruler is in inches.
My questions:

a) where would a Crayford Focuser like this one go? (edited after feedback) Should it replace part [A] or part [B]?
b) what is part [B] officially called? and what diameter is it referred to by? (I measured 1.9in but can't seem to find anything like that on the Internet)
c) how standard is this mount? Should I worry about the dimensions for what might replace Part [B]?
d) can anyone comment with potential focusers that you think would fit this build? I'm looking for dual speed and < $150

Thanks!

Note that I am aware these aren't the best scopes ever made but I do wish to keep it for a while.


texts: What was the length of the solar day 73 million years ago?

texts: How could this be estimated, or how accurately could it be extrapolated from a present-day measurement of the Earth's spin-down rate?


texts: There are 51 stars within 17 light years of the Earth (source). If one of these stars was to become a supernova, how would they effect the Earth?


texts: Are there any natural satellites in retrograde Sun orbit?

texts:
As I watch the video of the asteroids being discovered, all I see is prograde orbits. Their prevalence is fully understandable and justified, but the Sun gets interstellar visitors occasionally and I see no specific reason why it couldn't catch one into a retrograde orbit. They would obviously be a minority and likely not nearly as long-lasting as these in prograde orbits, but they should exist.

Did we observe any? If so, what sort of percentage of the whole are they? If not, why?


texts: Requirements to resolve position of Jovian Whistlers up to magnitude of Red Spot with amateur radio equipment?

texts:
Can I sense a bright star pointing an eight foot antenna towards it? is a while ago, but the discussion about this question I found very interesting. Please forgive me that I am not an expert in Jovian Whistlers but I tend to understand it as massive EM events on Jupiter.
I am wondering: Could we use a Yagi-Uda antenna or an array of it to resolve the source of the radio signals on Jupiter? For instance, we could have the Jupiter-disk covered by a square of $40 \times 40$ pixels, and the goal would be to figure out from which pixel the signal originated.
The idea behind that is that a cool research question for an (advanced) science project in school would be a question like "Did the lightning occur inside the Red Spot?" and Yagi-Uda antennas are fun to build, and do not require as much space as other antenna types.
References

Strong whistler mode waves observed in the vicinity of Jupiter’s moons


texts: Trouble understanding speed-dispersion in (elliptical) galaxies

texts:
I'm learning about LOSVDs (Line Of Sight Velocity Distributions) and I'm having a bit of trouble understanding the used terms.

As I understand, the LOSVD of a given (elliptical) galaxy is the density distribution of the LOS-velocities. The full LOSVD is difficult to find and it's easier to find 2 parameters of the distribution: $\bar v_{_{LOS}}$ and $\sigma_{_{LOS}}$ by fitting a (Gaussian) model to the spectrum of the galaxy.

I have a limited understanding of statistics, so I'm having trouble intuitively understanding what these 2 parameters mean later on.

I think that $\bar v_{_{LOS}}$ is simply the average value of the LOS-velocity for the whole galaxy while $\sigma_{_{LOS}}$ is the equivalent of a standard deviation.

Now later on in my coursebook, there's an explanation of how to get a LOSVDs for every point (/pixel) in the (projection onto the celestial sphere of the) galaxy by using 3D spectrography.
From this we can get a 3D dynamic model from our 2D kinematic model. In this newly found 3D distribution there's also 3 Sigmas, one for every dimension.

But here comes the part I don't understand:
The book is looking at the movement of stars in a spiral galaxy, the Milky way in particular.

We're trying to find a correlation between the age of MS-stars and their dispersion and so there's a dataset of a few stars in the neighbourhood of the sun with their dispersion in every dimension.

What is the meaning of dispersion in this context? How can a single object have a dispersion if it's a parameter of a density distribution?


texts: How much of an effect does the moon have on Earth's liquid mantle?

texts:
The gravitational pull of the moon is enough to create tidal forces of large bodies of liquid, i.e the sea.

I was having a conversation the other day about how to terraform Mars, and someone suggested that if Mars was to have a moon artificially placed there then its gravitational effects would help keep a liquid core flowing.

Does the moon therefore also have an effect on other large bodies of liquid on Earth? Does the moons gravitational pull affect in any way the flow of Earths liquid mantle?


texts: How can Earth-Sun Lagrange points L1 & L2 even be semi stable considering the moon?

texts:
I know that the Earth-Sun Lagrange L1, L2, and L3 points are not considered stable over longer periods, especially when compared to L4 and L5... But, with the moon orbiting the Earth in the general path of the ecliptic, it would seem that the first time the moon got in between the Earth and the object at the L1 or L2 points, it would be enough to perturb the object a good bit, and the next lunar month would be even worse, and so on.

It seems that if the moon was taken out of the equation, the L1 and L2 points would be much more stable. And, as a result of this line of thinking, I would think that the L3 point would be much more stable than L1 and L2, since the moon would be so much farther away and unimportant, so its effect could be ignored.

Am I wrong in thinking this? Is the moon's influence not large enough to be a major factor? And, just to clarify, I am talking specifically about L1 and L2 with respect to a large 3rd body, like the moon, instead of some idealized 2 body only system.


texts: Why is the Kuiper belt called like this?

texts: Since most orbits of KBOs are highly inclined, isn't it rather a "Kuiper cloud"? Most main belt objects are also more inclined than the eight recognized planets, but not as much as KBOs (and bodies beyond) which can orbit above the Sun's poles. Why isn't it called the Kuiper cloud then? Is it due to historical reasons (because the first discovered KBOs such as Pluto weren't as inclined as Eris for instance)?


texts: Anecdote of an astronomer looking for correlations in antipodal pairings of stars; what am I remembering?

texts:
The question Are the stars distributed in uniform distribution, on the celestial dome, with respect to brightness? brings to mind a different study that I vaguely remember hearing about a while ago, perhaps several decades. This is a summary of how I recall it, it may not be 100% accurate:

An astronomer was stuck in a situation for an extended length of time and in order to keep busy and do science used a small personal computer to do an analysis of star positions, focusing on pairs that were diametrically opposed; at nearly antipodal points, in order to see if there was a deviation from other pairs of points that were not antipodal.

One possible effect that might cause such an anomaly was that the universe was small and closed and light from a star might reach us from both the "short" and "long" part of a great circle drawn on a 4-sphere (or torus?), thereby appearing to us at antipodal points of the celestial sphere, a bit like how amateur radio operators with directional antennas with good rejection ratios can sometimes receive the same signal from both directions along the great circle containing both sites.

Does this sound familliar to anyone? Was it ever published? I've tried googling several ways but lack the proper search terms.


texts: I know that you can calculate the distance to another star for example, by using the phenomenon redshift to determine how much it has redshifted because of the expansion of the universe. But how do you know what the original frequency/wavelength of the light was? (And if all of my above statements are wrong, how do you measure the distance?)


texts: Terminology: Is there a name for the points on the surfaces of tidally locked parent/satellite bodies that always face each other?

texts:
This is purely a question about terminology, one that has eluded my googling efforts.

When a satellite and its parent body are tidally locked to each other, there is (in an ideal case) a single point on the surface of the parent body and on the surface of the satellite body that remain the closest two points on the two bodes at all times.

Is there a specific name for those two points? If so, what is it? I think "antipodal points" is descriptive, but I wonder if there is a more specific term.

Thank you!


texts: Why a blue spot in some hubble images of NGC 4302?

texts:
In some images with coloring like in this one:

There is a noticeably bluer region in the "bottom left" of the image, and the bottom part of the galaxy on the left. Being an inexperienced galaxy viewer, I can't figure out what to make of it.

Is the spectrum of that region more complex than it seems? What is it?


texts: Seeming conflict between most distant objects and age of universe (both estimated)

texts:
There was a recent article on bbc.com for laypeople like me titled "The mystery of how big our Universe really is", which prompted me to post the following question in their comments section of Facebook, which so far has received no answer, so I am reposting it here in the hope of a better result:
"I have a simple question, which probably has an equally simple answer: if "the furthest visible regions of the Universe are estimated to be around 46 billion light years away" but at the same time "the Universe burst into existence an estimated 13.8 billion years ago", wouldn't the photons (or whatever) that we observe coming from those distant objects have had to have begun their travel towards us long before the universe existed?"


texts:
This excellent answer to the question What is the physics of the “spinning dust” contribution to Cosmic Microwave Background measurements? Is well sourced. The abstract of one of the papers there, Purcell 1975 Interstellar grains as pinwheels begins with:

  Contrary to the assumption usually made in theories of grain alignment, the rotational energy of an interstellar grain is likely to be very much greater than 3/s kT, where T is the gas temperature, or the grain temperature, or any other temperature in the system. Any more or less permanent irregularity of the grain’s surface with respect to accommodation coefficient, distribution of H-H recombination sites, or photoelectric emissivity will result in an unbalanced torque capable of spinning the grain up to high angular velocity. Such a grain is, in effect, the rotor of a heat engine.

In the body of the paper, the first example given is of a sort-of gedankenexperiment involving a hypothetical Crookes radiometer with the gas removed. The author explains that it would spin up to high energy (without saying how):

  [...] The kinetic energy of the rotating paddle wheel is now enormously greater than it would be if it were immersed in, and en equilibrium with, a 6000 K radiation field, for then it would exhibit only a residual “Brownian rotation” appropriate to that temperature. So the wheel is really a heat engine, depending on the difference between the temperature of the vanes and that of the radiation field to which they are exposed.

Without the gas to cool them, I suppose that after the whole thing reaches equilibrium temperature the black sides radiate more strongly than the white sides and so there should always be a torque, and the final speed and kinetic energy would be limited only by friction and other practical losses.

This excellent answer to the question What is the difference between gas and dust in astronomy? points out that there is no real distinction between the two, but that they are limits in size, gas particles being small and dust particles being large(r).

So if dust can become pinwheels, then I have a hunch that very very large molecules can do this too, and if so, then so can smaller molecules. All you need is some mechanism for torque. Molecules can certainly have a non-uniform distribution of  H-H recombination probability just as dust can, and they can have some regions that are concave and some that aren't.

This makes me wonder if these gas molecules no longer obey the Equipartition theorem of thermodynamics. The spinning dust certainly doesn't.

Question: How can spinning dust and by induction, my hypothetical spinning molecules avoid conforming to the equipartition theorem? Why doesn't the equipartition theorem disallow spinning dust?

Source borrowed from here

Bonus points: Is that little arrow ($\omega_{rot}$) pointing the wrong way?


texts: Why is the Solar Helical (Vortex) model wrong?

texts:
Could someone give a lay description of why the Solar System's depiction of motion as a vortex is wrong?

ref: Youtube video The helical model - our solar system is a vortex

Please don't take for granted that because of my previous question about why planets or asteroids don't have cometary tails, that I believe the 'tails' shown in the video are real. I do not. I know that the tails as shown are simply a graphic representation of the path 'flown' through space by the planets. It seems that the solar system we inhabit has to be moving through interstellar space in some fashion. If the paths of the planets could be shown in a video, it seems to me (a lay person) that there would have to be motion 'forward' in addition to the eliptical path around Sol.


texts: How to find how many degrees from the beginning to the end of the sunset?

texts:
According to what I what told here there is 0.5 degrees from the beginning the end of the sunset.

Unfortunately I didn't understand two things:

  1) How can I prove in the simplest way that there's 0.5 degrees only
  from the beginning to the end of the sunset.

  2) Why it is not adjust to my past observation on these phases (which
  was much longer (range from 5-7 minutes). which makes it between 1-2
  degrees.


texts: Where can I find a solar eclipse calendar compatible with Google Calendar?

texts: For planning purposes it would be good to have a calendar of upcoming (at least some years) solar eclipses. The closest thing I've been able to find has too many other events in it to be useful. A similar calendar is no longer reachable.


texts: How can comets have tails if there's no air resistance in space?

texts:
I understand that solar radiation causes material to vaporize out of a comet into dust but why does the dust then trail behind the comet like a "tail"?

Assuming gravity is the only applied force acting on the comet, shouldn't all of the material, including the dust, be travelling at the same speed due to conservation of momentum? What causes the dust to travel slower than the comet's nucleus? In other words, why does the dust form a "tail" and not a "cloud"?


texts: Requirements for a satellite/planet to be tidally locked to a planet/star

texts: The Moon is tidally locked to the Earth, the Four Galilean are tidally locked as well, and the recently found planetary sistem TRAPPIST-1 has seven tidally locked planets, but Venus or Mercury are not. Why? Is there any conditions or analytic formula which says if a small body will be eventually tidally locked to the major body?


texts:
the more concentrated the mass, the more powerful is gravity? or does it depend exclusively on the total mass? example: 1 $\small\mathsf{cm^3}$ material A = 1000 kg, 20 $\small\mathsf{cm^3}$ material B = 2000 kg
which of the two generates more powerful gravitational force?

I apologize for my English, I am not a native speaker, and I used google translate


texts: How far from Betelgeuse is its habitable zone?

texts: Betelgeuse is a red supergiant or hypergiant in the constellation Orion. It is one of the biggest and brightest stars seen from the Earth, and is expected to soon go supernova. The habitable zone, or Goldilocks zone, is the distance from the parent star where the temperature allows for liquid water to exist on a planet's surface (in case the planet has an atmosphere of at least ~0.0061 atm (triple point of water)). How far is the habitable zone of Betelgeuse, in astronomical units? For comparison, the Sun's habitable zone is at about 0.7-1.7 astronomical units (other than Earth, Venus and Mars are on its rims).


texts:
If you trace two particles' world line backwards in time, according to current theory, both objects should converge at big bang.

Would both objects arrive there simultaneously?

Another way of asking the question:

Is there any evidence that supports the idea that new matter is "introduced" into the universe as a continuous stream, as opposed to everything coming from a singularity both in time and space?

The analogy would be from the inside of a black hole. For each particle in that singularity, they would all have a world line that starts at the same spacial coordinate, but the particles would not be "introduced" into the black hole simultaneously. You can argue that time stops having meaning in a black hole, but that's only true for an "outer observer" in my reasoning.


texts: For stars around 13th magnitude, are their entries in various catalogues usually linked, or does one do this manually?

texts:
@MikeG's speedy answer to "What star was the asteroid 2014 MU69 predicted to occult on 2018-08-04?" lists three names for one 13th magnitude star, from three catalogues; 2MASS, UCAC4, and Gaia DR2.

My question could be slightly subjective, but I believe that someone familliar with the topic will be able to provide an authoritative answer.

For stars around 13th magnitude that do appear in all three, are they electronically linked between catalogs in some automated way, or do you pretty much check each of them, decide the objects are the same, and then make note of your conclusions if you are writing something up?

2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
UCAC4 Catalogue (Zacharias+, 2012)
Gaia DR2 (Gaia Collaboration, 2018)


texts: If planetary rings are maintained by staying within the Roche limit, how is it that  J1407b's rings extend so far? Surely the gravity and tidal forces should be low enough to allow a moon to form.


texts:
In a recent astrometry question in Space SE I needed to check Proxima Centauri's position and the article contained the image below.
I've saturated the colors, cropped it, and added a "retroactive coronagraph" (black dot in the center) to show that the color scheme of the dozens of small radial striations is similar to that of the four major spikes coming from the vanes holding Hubble's secondary mirror.
Hubble's point spread function should contain diffraction from various things including:

the primary aperture
the central occlusion of the secondary
the four vanes holding the secondary
the three holes in the mirror (!!!) used to mount it

but there must certainly be more items in this list.
Since this is an intensity map rather than amplitude, there is no phase information so I can't perform an inverse Fourier transform to see what's causing this, so instead I'll ask here.
Question: What produces all of the small radial striations in this very overexposed image of a star by Hubble's WFC2? (the four big ones are from the vanes)
It seems that this image can be found at https://esahubble.org/images/potw1343a/ which says that this image contains two wavelengths: Optical R 675 nm and Infrared I 814 nm. Since diffraction is purely chromatic we can indeed expect any diffraction effects to demonstrate strong dispersion (i.e. those colorful two-and-not-three-color patterns in this image).

Source click the right one for larger


texts: Assumption of our universe being the surface of a 4-D sphere in order to describe relative uniformity in the cosmic microwave background?

texts: It seems as if this is necessary to assume in order for the CMB to make an sense. Otherwise, an instantaneous infinite Euclidean space model would work. The issue is, with any visualization of the Big Bang, let's take a Big Bang visualization in 3 - dimensions, the radiation emitted from the creation of hydrogen atoms would not necessarily be all around us, as it is. Our position in space would dictate what we would observe in terms of background radiation.


texts:
I have a more mathematical background but have always been interested in astrophysics, cosmology, astronomy, etc. There are a lot of popular books on the subject, but I am getting more interested in looking at a more rigorous approach to it.

What are the best introductory textbooks on the subject? (Calculus+ and assuming introductory courses in Mechanics and Heat and Electricity and Magnetism, etc.)


texts: How close would quasars have been to each other?

texts:
My understanding is that quasars are the early phase of formation of galaxies.

At the time quasars were forming, the universe would have been smaller and therefore things closer to each other.

About how close would quasars have been to each other?


texts: So as I understand it the source of most geothermal energy on Earth is due to radioactive decay. Jupiter is the most massive planet but if the bulk of it's mass comes from gaseous elements instead of heavier ones like uranium or whatever than it being the most massive might not necessarily mean it has the most radioactive material.


texts: Refinement to sun/wristwatch estimate of N/S direction

texts:
As discussed in a 2015 question here
and shown in a 2013 illustration in Primer Magazine,
it's possible to estimate the north/south direction on the ground using a correlation between the hour hand of an analog watch and the azimuth of the Sun.

As Wikipedia:Analog watch suggests, the relation between hour angle and azimuth depends on latitude:

The method functions less well as one gets closer to the equator.

Also the Sun's declination affects its azimuth around 6 AM or PM.
Assuming that the observer has already determined the local apparent solar time,
what further correction could they apply to estimate the north/south direction more accurately?
I expect that this would be a function of the observer's latitude φ, the Sun's declination δ, and the Sun's hour angle h.


texts: What fractions of matter are retained and displaced by quasars?

texts:
Following on from a query concerning quasar mass, what fraction of matter is ejected compared to matter that is lost to the BH?

Is there a graph that describes the fractions of the mass of quasars which contribute to disk radiation, plume radiation and black-hollowed? (sorry I can't think of a word because accretion and accretion disk is confusing)


texts:
Aristarchus estimated the relative distance of the Sun and Moon by
  observing the angle between the Sun and the Moon (α in the diagram)
  when the Moon is exactly half lit. Angle β must be 90° for the Moon to
  be half lit. By observing the angle α, he could then set the scale of
  the triangle and thus the relative lengths of the sides. (Sizes and
  distances are not to scale.)

Does half-lit mean exactly 45 degrees?


texts: How long after the Big Bang would CMB radiation have been between 273 and 373 K?

texts: Cosmic microwave background radiation is 2.7 K today. When would it have been between 0 and 100 C?


texts: Why do spectroscopic binaries have approximate circular orbits?

texts:
So for an assignment I have to answer the question what I can conclude concerning the shape of the orbit, knowing that it is both an eclipsing and spectroscopic binary.

Now, I think the answer I supposed to be that, since we know it is a spectroscopic binary, the orbits will most likely be circular. And I've read on various places that this is true, because of the tidal forces that cause for circular orbits. These tidal forces are quite strong, because apparently spectroscopic binaries are quite close to each other.

So here is where I lose it, I don't understand why spectroscopic binaries should per se be close to each other. I have considered that this is because they are not visual binaries; if they were visual binaries we would be able to tell them apart. But we can't tell them apart, so they are not visual binaries. From this you could conclude that this means that they are close to each other, but I think it could just as well mean that they are very very far away, and that that is the reason we cannot tell them apart. So I can't find a proper explanation for this.

Many thanks if you can help me out!!


texts: Was stargazing last night and spotted an object traversing the sky from roughly East to West. It had a bright light that was intermittently flashing but with three different intensities. The "third" flash was really bright like a camera flash. Never seen a plane with lights this bright and was not a satellite (like any that I've ever seen), nor was it the ISS as that was not in this part of the world at this time. Any ideas anyone?


texts:
How many craters on Mars have ice in them and which ones have permanent ice? I could only find Korolev and the unnamed one - strange for such a remarkable crater.
http://www.esa.int/Science_Exploration/Space_Science/Mars_Express/Water_ice_in_crater_at_Martian_north_pole
One would expect all such craters to be famous already.
Which such crater is closet to the equator?


texts: Could evaporating hot Jupiters have metallic hydrogen on their surfaces?

texts: Jupiter is believed to have metallic hydrogen in its core. And gas giants that migrate to become hot Jupiters are believed to evaporate, have their atmospheres blown away by their nearby star. Can metallic hydrogen (or some other exotic materia) be detected on the surface of such a planet, or does it require high pressure to exist? What happens to the metallic hydrogen? Does it release much energy if and when it is transformed?


texts: help needed with inactivated option for wcs in SAO Image DS9

texts: Another trouble in SAOImae DS9: with the Rotation Measure catalogue I have just downloaded: “Rotation measure image of the sky (Taylor+2009)”, even if I have set up the WCS to galactic before the catalogue loadind, I obtain the table file with another WCS (and the option to chane WCSto another one is inactivated); then I can not obtain from it the galactic longitude and altitude needed for my report..


texts:
I'm trying to decode the Variable Types of stars in the Hipparcos Library.  I see there are a number of stars for example :-

EA/AR - Does that mean the star is both a EA(Eclipsing Binary Star) and AR (Detached System of AR Lacertae) type variable star ? e.g. HIP112009

Whats the difference between EA (Delta Orionis) and EA: (Omicron Puppis) type stars ?  What does the colon signify?

Edited - So on line, 2575 of hip_va_1.dat, the following line has a Variable Type of EA/AR, what does the slash signify.

112009| |F5V         |P|EA/AR | | 9.016| | 9.65 |  -1.7|   2.96850  |  -4.1|8501.4698|4|A| |W_Gru       |   2.96850|2430132.15| 9.40|10.00|P|R

What does the (+) sign signify in a Variable Type e.g. SR+ZA (HIP36623)

I really like to understand what the significance of the +, / and : mean


texts: How many wandering black holes are there in the milky way?

texts: I have attempted to research this topic due to curiosity, but I have yet to find a credible answer, the best I could find is this article, which has no listed sources, and the author has also done similar "click bait" style headlines with little to no evidence.


texts: Blackbody curve in RGB for objects less than 1500 K?

texts: We know the colors of stars that have a temperature greater than 1000-1500 K, shown here. However, I am wondering about those stars/brown dwarfs with surface temperatures of less than 1500 K. Is there a blackbody curve for these temperatures, and are there any set RGB values for them?


texts: Decreasing rate of Earth's rotation: where does the power go?

texts: I understand from Wikipedia that, "The length of the day, which has increased over the long term of Earth's history due to tidal effects, ...".  If the angular velocity of the earth is decreasing then the stored rotational energy is decreasing.  Where does the power/energy go?


texts: Relationship between galactic rotation and stellar lifetime

texts: I have read that the lifetime of stars is much less than the period of rotation for a spiral galaxy. For example https://www.coursera.org/learn/evolvinguniverse/lecture/wlYhz/the-origin-of-spiral-arms the professor uses the existence of young blue stars whose creation is triggered by a persistent density wave to claim that the stars lifetime is less than the rotation period. I am not satisfied with this. Given the enormous timescale, how do we know this?


texts:
The super massive black hole at the centre of Milky Way has a mass of about 4 million times that of our sun. Is this enough to keep the entire galaxy together, is it this black hole which keeps the galaxy together? I did read that there are also galaxies that don't have a super massive blackhole, what keeps those galaxies together?
I did ask this question at space stackexchange but it got closed because it was posted at wrong place.


texts: Why stars are not visible in photos of Planets from space

texts:
The night sky from many parts of the Earth, away from any city lights and pollution, is full of starts, mostly the stars of the Milky Way Galaxy is what fills the night sky densely.

In many photos of outer space capturing the planets of the solar system there is always the black background, not even one single star to be found (I searched for photos of Saturn, Jupiter and even Earth, didn't find any peck of light around it which may be a star)

My question is, why this happens? I've told myself regularly that there can be three reasons for that

The exposure of the camera is too low to capture the stars that might luckily be in the background of a planet's photo.
The stars are too spread out each other that in most of the photos there might not even be a star in the dark space behind the planet.
The telescopes taking the photos aren't powerful enough to take the photos of the stars clearly (very much related to the 1st point, but instead of the exposure of the camera I believe it is the telescope that's not capable or maybe made for that)

I have tried looking for answers but didn't find much, maybe if the photo was taken from an angle from where the Milky way galaxy is just right behind the object i.e planet here, we may have seen some twinkling in the background? Or am I wrong?


texts: Real images (not "artist concept") by NASA

texts:
Does NASA make publicly available real images of missions in an organized manner? What I mean is: Say I'm looking for the images taken by New Horizons - a simple list of the images in chronological order. Even if it's not a comprehensive list, at least all of those made publicly available. Does that exist?

To be clear - I'm not interested in "artist concept". On the other hand - "false color" pictures of infrared etc. are fine.


texts: Expansion of Space in the Very Distant Future

texts: Given a nearly infinite timeline, would the expansion of space eventually cause individual particles to become "unobservable" from one another due to the vast distance and increasing rate of expansion with that distance.


texts: Is there anything special about our position in the galaxy thats necessary for life?

texts:
It's a fact that our planet's location from our parent star (among other factors) is of crucial importance to the existence of life. But can our position in the galaxy be considered a factor when considering potential planets to carry life?

In any given position in the galaxy for instance there are distinct radiation levels, distinct gravitational effects, space-time, etc. These factors could contribute to the existence of life on earth or even its continuity, wouldn't they?


texts: Definition question: continuum-subtracted spectrum

texts:
What is the exact definition of a "continuum-subtracted spectrum", such that one finds in SDSS spectra?

http://classic.sdss.org/dr7/dm/flatFiles/spSpec.html#specmask

Is there a definition somewhere? Thanks


texts: Are there are any photographs online that approximate what the Milky Way looks like to the unaided eye?

texts:
I love seeing all the long-exposure photographs of the Milky Way, but I'm going to Cherry Springs State Park in Pennsylvania next month, and I'm wondering if there are any photos that approximate what our galaxy looks like to the unaided eye. I'm really just trying to set my expectations, because I know it's not going to look like what you generally see in long-exposure photos. (Naturally, people are generally showing off their carefully crafted and enhanced images, rather than trying to show what we actually see with our eyes.)

Edit: I did find this post, which, according to the author, comes close to approximating the view with the unaided eye. For those that have seen the Milky Way from a Dark Sky site, though, I'd be interested to hear your opinion on the relative accuracy of this image.


texts: "True" motionlessness - red shifts and CMB

texts:
This is a question I asked years ago in an astronomy course, but to which I never got a straight answer. Please feel free to correct me if any of my assumptions here stray from facts. It goes like this:

To any observer, at large enough scale the universe appears to be receding in accordance with Hubble's constant, which causes the light that reaches them from objects such as distant galaxies to experience a red shift. Technically, though, if the observer is standing on a planet, they are in orbit moving in a particular direction at any given time, and so the distant galaxies in the direction of that should have a(n extremely) slightly smaller red shift while those in the opposite direction should have one slightly more severe. This means that, relatively, the galaxies behind are receding at a faster rate from the observer than those in front.

If the observer were to possess instruments of sufficient precision to determine the discrepancy in the red shifts and the means to adjust their velocity in any necessary way, would it be possible for them to equalize the red shift in every direction and thus be motionless in respect to the expansion of the universe itself? The cosmic background radiation might be a good reference for measuring red shifts in every direction, since it should be equally distant everywhere and very, very red shifted.


texts: I have decided to do undergraduate thesis on "estimating photometric redshift" or something related to this using machine learning. Reading previous papers, I have come to know that work has been done on this topic using support vector machine, artificial neural network, Bayesian approach and even deep learning. Can you suggest what are the  modifications or additions that I can possibly make?


texts:
If I have understood correctly, stars form in big clouds of gas and dust that are pulled together by their gravity. And the stars are often ignited when something disturbs the cloud, such as a passing massive body, or a wave like the once which create spirals in galaxies.

Are there any phenomena on earth that behave like these clouds, which could be used to illustrate and explain star formation? Something people might be familiar with and which you could show a video clip of?

I'm thinking about some gas that is disturbed then takes solid form? Or something completely different which looks similar?


texts:
According to the ScienceAlert article Mysterious repeating radio signals have been detected coming from outside our galaxy, based on the Letter to Nature "A repeating fast radio burst", there has been a series of signals detected that the article reports that scientists have not seen before.  From the article:

  Astronomers have detected repeating blasts of radio signals coming from deep space. These short-lived signals are known as 'fast radio bursts' (FRBs), and although we've heard them before, they were always thought to be one-off events coming from random locations. But for the first time ever, researchers have now heard repeating signals, all emanating from a single unknown source outside our galaxy.

Emphasis mine

In the Letter to Nature, it is hypothesised that the source could be that

  these repeat bursts with high dispersion measure and variable spectra specifically seen from the direction of FRB 121102 support an origin in a young, highly magnetized, extragalactic neutron star

This leads me to the question:

How could a neutron star cause repeated 'fast radio bursts' detected coming from outside of the galaxy?


texts:
I know there are stars much larger than the sun and that we've discovered "small" black holes, so, let's say that the Pistol Star, which is at least 80x the mass of the sun, and the smallest known black hole, XTE J1650-500, around 4x the mass of the sun, collide. Would the pistol star "eat" the black hole, or is there some property that a black hole has that is immune to the star's power?

I would think that the star would turn the black hole inside out, and like explode or something, but there's also the possibility that once a black hole rips space time the way it does maybe that can't be easily undone. I'm assuming the matter within the black hole isn't going to magically pop back into existence either as it's been broken down into it's most basic parts.


texts: What limits the use of the H-R diagram to measure distance (main sequence fitting), what distances is it useful for?

texts: Is it only possible to measure objects that form around the same time? Is it possible to measure clusters from distant galaxies other than our own?


texts: Did we discover 10 or 12 new moons of Jupiter?

texts:
I saw multiple news sites reporting that a team discovered 12 new Jupiter's moons:

c|net - Twelve new Jupiter moons found, including one reckless one
Discover - Jupiter’s Got Twelve New Moons — One is a Bit of a Problem Child

While some other news sites claim that they found only 10 new moons:

nature - Jupiter has 10 more moons we didn't know about — and they're weird
EarthSky - Astronomers discover 10 new moons for Jupiter

Why does the number of moons differ in the different articles? Did the team now found 10 or 12 moons?


texts: Do massive bolide entries coincide with meteor showers?

texts:
Are exceptionally large, bright, meteors (such as Tunguska or Chelyabinsk) more common during common meteor showers like the Perseids, or the Leonids?

If not, why is that? Do the meteor showers simply not carry any more massive pieces?


texts: How do I remove fungus from a telescope mirror?

texts:
I use a Celestron NexStar 130SLT Azimuthal Newtonian Reflector. I noticed that recently, due to the wet season, some tiny spots of what appears to be fungus have appeared.

Having taking appropriate precautions to prevent the same in future, how can I presently get rid of these fungi colonies on my primary reflector?


texts: Regarding the age / size of the Universe

texts:
I believe the current estimated age of the Universe is around 14 billion years.  I just read in another post on here that the diameter of the Universe is around 90 billion light years.  This doesn't make sense to me, shouldn't the radius of the Universe be equal to it's age?  Unless the explosion caused particles to spread out faster than the speed of light? Is that possible? I thought the speed of light was a speed limit that nothing can exceed?

Sorry if there is something I'm not understanding here or if I'm trying to compare apples to oranges.  I'm only an astronomy enthusiast.


texts: Of course it is not exactly known how big the universe is, but I thougt that the universe is about 100 billion lightyears in diameter. But if that is true can you also make an estimation of the volume of the universe?


texts: What is a UBV source for stars or stellar objects?

texts: I was using the V19(VizieR) catalog and the main identifier of the stars was in terms of an 'ID' number which they mentioned as being 'Star number in UBV source' I have no idea what that means. How would I find a more common identifier say  HD/HIP/HR number?


texts: What are the conditions for an ionosphere to form?

texts:
What are the conditions for an ionosphere to form on a given planet?

Factors I could think of are

Class and age of the star the planet orbits
Distance to said star
Density and composition of the planet's atmosphere


texts:
Are there pictures that contain high enough dynamic width, so that amateurs can use fourier spectrum analysis to observe the redshift themselves?

I know pictures usually are encoded using RGB and/or CMYK and therefore lose a lot of color information from a physical perspective, but I'm wondering if there exists a picture format that could store that much information, and if so, could amateurs then use fourier spectrum analysis to actually see the red-/blueshift?


texts: Why does the adiabatic exponent decrease at the ionization zones?

texts:
The context is the ionization zones in stellar atmospheres or interiors, the Sun, for instance.

The adiabatic exponent is the heat capacity ratio:

$$\gamma = \frac{c_P}{c_V} = \frac{C_P}{C_V}$$

And, for an ideal gas, it can be shown that  $\gamma=(f+2)/f$ where $f$ is the degrees of freedom. Resulting in $\gamma\approx1.66$.

In most of the extent of the solar interior, $\gamma\sim1.66$ but it presents dips when ionization is present, like the H, HeI and HeII ionization zones. My question is why does this happen?

I thought it could be because when ionization, the heat is used to ionize atoms instead of increasing the temperature and, therefore, the heat capacity would be larger in those regions. But, because the $\gamma$ is the ratio between the heat capacities, the argument does not work.

Any suggestion?


texts: This isn't a question so much as a request. I'm looking for an image of the Orion constalletion I could use, but I would like to have a very high resolution HD image, whereby a lot of the deep sky objects can be seen to high detail other than just the 7 hourglass stars of the belt and four corners. Would love to have perhaps the Orion nebula (Messier 42), the Horsehead Nebula (near ζ Orionis), the emission nebula NGC 2174, Messier 43 and Barnard's Loop.


texts: Why is Earth's rotation considered fast?

texts:
I'm currently reading Atom (Krauss book), and in chapter 11 there is this text:

  This collision can explain many things, including the high rate of spin of the Earth (the grazing collision would have twisted it around like two football players who collide while running in opposite directions).

This is in reference to a proto-planet hitting the early Earth, which led to the formation of the Moon.

Why is Earth's spin considered high rate, given that Mars' spin is pretty similar without having a big moon? Or have I misunderstood what is being referred to?


texts:
StDr56 is a newly discovered planetary nebula (maybe). link1 link2
It was found by amateur astronomers Marcel Drechsler and Xavier Strottner.
According to the above articles, it's pretty big:

With an extension of 44 x 36 arc minutes, StDr56 is not only the
largest probable PN in the region, but in the night sky it covers more
than half of the area of the famous Triangulum Galaxy M33, which is
located only a few degrees from Strottner-Drechsler 56.

Or

It’s about the same size as the full Moon on the sky.

So, if it's so big, why was it only discovered now? Wouldn't it make sense if all large objects were already discovered a long time ago?


texts:
All I could find on the internet was about how stars vary in brightness depending on their distance to Earth, temperature, type of star...

But my question is, can a star can change its brightness.


texts: How significantly does the intensity of a meteor shower diminish after the peak?

texts:
Every August 12 I take the family to the desert to enjoy the Perseid meteor shower. This year there will be an especially bright moon rising before 1:00, which is when the shower really peaks. Last year especially was a disappointment as the moon did not set until 2 or 3 in the morning, and there was little to see until it did, so I don't want to disappoint the family two years in a row.
If I wait a day or two (depending on scheduling conflicts), so that the moon might rise later and be a bit less full, how much would that be expected to reduce the intensity of the meteor shower? I've found this paper discussing meteor shower activity profiles, but nothing that I could apply to this year's Perseid shower. This question also seemed relevant, but it does not contain the detail need to determine if waiting a day or two would significantly reduce the visible activity.
Is meteor activity significantly reduced a day or two after the peak, for purposes of recreational observation by experienced observers?
I understand that nobody can predict (space) weather. But the Perseid shower typically persists for two weeks after the peak. Is that a linear drop off? Exponential? Does the shower typically loose 5% of its intensity in two days, or 50%, or 95%?


texts: Is it possible to see mercury transit "clearly" by the naked eye?

texts:
I don't have a telescope but I'm interested in seeing events like eclipses and transits.

I'll use the atmosphere as my big natural lens. So I'll watch the upcoming mercury transit at the sunset time where the sun looks bigger than usual. Will this make the black dot of mercury bigger or big enough to be obviously seen by the naked eye? I've never seen it before.

Event: https://www.timeanddate.com/eclipse/in/egypt/cairo


texts: Are there any ready-made machine-learning datasets of photometric data that are easy to use?

texts:
Disclaimer: I have a Machine Learning/Computer Vision background, not Astrophysics, so please be a bit patient with me. If you feel like this question is too far out there it would be great if you could direct me to meta or some bulletin board/mailing list that you think is a better fit. :)

Important: I am talking about "datasets" in a machine learning / computer vision manner. I do not mean "the entirety of SDSS data" but rather a common set of inputs and (for supervised learning) associated outputs for a specific problem.

I have been looking for a while now to tinker with some problems in Astronomy/Astrophysics and I keep finding the same kind of advice when it comes to datasets:

Go to SDSS/GalaxyZoo, they provide downloads for their data.

Now, while that is true, none of these datasets are anywhere near as accessible as datasets in Computer Vision. "Over here" a dataset is mostly a bunch of (huge) tar files with jpeg images and some .mat or .csv for annotations. The process of getting started is pretty straigth-forward: unzip, load annotations into memory, start sciencing (or usually, applying wild heuristics). Obviously I simplify slightly, but you get the idea.

Now, to stay with the example of GalaxyZoo, the object coordinates, annotations and computed features are conveniently presented as a csv, fits or voTable file. But to get the actual visual data (if I understand this correctly) I have to, for each annotated galaxy...

determine a bunch of .fits urls from the coordinates in the annotations (one per band and galaxy) and download them (~12MB/file)
download the containing "tiles" from the SDSS servers (in each of the bands I'd like to work with)
extract the patch containing the galaxies (with the aptly named sExtractor)
possibly apply some non-trivial corrections/transforms to the image patch

I tried following along this procedure with a semi-randomly chosen paper from arXiv for which there is code available here and without wanting to make any statement about said paper/code, this process is not comparable to the simple workflow I described above, and I would probably download some TB of data, were I to use the entire dataset, of which only a tiny fraction would actually be useful.

The resulting dataset only occupies a few GB of space, and has a very simple nature: ca 50000 images, with associated labels -1 or 1, for stars or galaxies. The data preparation pipeline, while one might also want to optimize it for this task, has next to nothing to do with the "real" work being done in the paper, and so having the reader go through those steps makes little sense to me.

So my questions are:

Did I understand the workflow correctly? Maybe I'm just following the wrong kind of paper, or I really just overlooked a big red "Download Here!" button? Otherwise:
Did anyone go through these steps and make the resulting dataset available? Or are there good reasons not to do so? From the size of the sExtractor manual I gather that the choice of parameters in this processing step might significantly influence the outcome of any downstream method. So...
Are there any sane/agreed-upon default settings for the preprocessing pipeline? While for any given method, applying extra "magic" in the preprocessing might make sense, having a default set of parameters would make dataset creation and a bunch of other things much easier, no?

I am sorry if this is a naive newcomer question. And for the long post. :P
As a meta, I would add tags "dataset" and "programming" or some such, but I lack the reputation.


texts: Multiple apparent lunar eclipses last night

texts:
I saw something very strange last night that I'm hoping someone can help me explain.
I was sitting on my roof with a night-cap at about 3am last night. The moon, about the last quarter, was low in the sky in the east. Something truly strange happened. The moon started to disappear. There were clouds in the sky, but it wasn't this, I'm quite sure. The moon was being eclipsed. The dark part (at the top of the moon, as the moon was lying on its back - a sailor moon? That's what it looked like, like you could sit in it.) grew, the crescent closed, until it was just a spot of cream colored light and then winked out entirely.

I was a bit amazed - I'd stumbled onto my roof in the dead of the night and by complete chance witnessed a total lunar eclipse? I sat there in awe, staring. That's when the moon started to reappear, in the reverse procedure as its disappearance. Becoming a gleaming point of light, then a thin crescent, then the quarter moon which I suppose is what it is "supposed to look like."

AND THEN IT HAPPENED AGAIN. The moon was eclipsed and revealed again.

Both times the whole process took about 6-10 minutes. I'm not sure if this cycling continued, it was late and I had to sleep. I checked the NASA site for eclipses for my time and location, but no.

Has anyone ever seen anything like this, or have any plausible explanation as to what is going on here?